JPH0524201B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing strong steel, which is suitable as a material for large structures used in low-temperature environments such as extremely cold regions, at low cost without requiring expensive alloying elements or special equipment. be. In recent years, the progress of the various industries surrounding us has been remarkable, and along with this, various resource development activities, such as the development of underground resources and the development and cultivation of marine resources, are becoming more and more active. It's deepening. In order to support these activities,
Advances in steel structures used in various fields
Development cannot be overlooked, but due to the need for higher efficiency in development activities and the need to operate in undeveloped areas with harsh natural conditions, steel structures have begun to show a tendency to become even larger. It has become necessary to use them in harsh environments such as on the ground. By the way, steel is generally known to exhibit a low-temperature embrittlement phenomenon in which toughness rapidly deteriorates at low temperatures.For this reason, steel materials for large structures used in extremely cold regions are There was a strong demand for a material that not only had high strength but also showed excellent toughness even in extremely cold temperatures. Conventionally, in order to strengthen steel in order to meet these demands, Ni steel was selected as the basic component steel and heat treatment was applied to it to create a fine tempered martensitic structure and a fine reverse transformed austenite that precipitates during tempering. This was usually achieved by producing a mixed structure of fine tempered martensite and fine tempered bainite, or by producing fine reverse transformed austenite. However, the strong steel obtained in this way had the extremely disadvantageous problem that it was necessary to add a large amount of Ni, and therefore the cost of the steel material was inevitably increased significantly. Therefore, the present inventors first proposed that ``Even if the steel does not contain large amounts of expensive elements such as Ni,
``If the cooling conditions from the austenitic state are limited to specific conditions, high strength and excellent toughness will be exhibited at the same time.'' Based on this knowledge, he proposed a method for manufacturing strong steel in JP-A No. 57-89424. , has provided strong steel with excellent properties at a low price, but the method proposed in JP-A No. 57-89424 also requires ``changing the cooling conditions for each component of the steel.''"The manufacturing process is somewhat cumbersome because of the need to do so." From the above-mentioned viewpoints, the inventors of the present invention have developed a steel that does not contain the addition of Ni and has an inexpensive composition, that does not require any troublesome work, and that is sufficient for large structures used in extremely cold regions. As a result of our research to find a simple and low-cost method of manufacturing strong steel that can be applied to
We came to the knowledge shown in (d). That is, (a) The structure of the steel can be changed to an extremely fine tempered martensitic structure without selecting Ni steel as the basic component steel and using the fine reverse transformed austenite that precipitates during tempering, as in the past. Or, by simply forming a mixed structure of ultra-fine tempered martensite and tempered low-temperature bainite, it is possible to obtain a steel material with appropriate strength and toughness that can be used as a steel material for large structures in extremely cold regions. (b) It is generally known that quenching using a rapid heating method such as induction heating is effective for refining the grains of steel.
In steel that simultaneously contains a specified amount of one or more of the Nb and Ti components, even if heated at a slow heating rate of 1°C/sec or less, such as in an electric furnace, the Ac ₃ point ~ [Ac ₃ points + 200℃]
When the quenching process is repeated at least twice after heating to a temperature of N) or Nb (C, N) does not destroy the lath of martensite or bainite during heating during the next quenching until the temperature reaches a high temperature just before transformation to austenite, thereby preventing a decrease in dislocations. Therefore, austenite is
In addition to former martensite grain boundaries, it is also generated from lath boundaries to form a fine structure, and by quenching, an even finer low-temperature transformed structure (martensite, low-temperature bainite) is produced. Furthermore, Nb and
Since Ti prevents austenite crystal grains from becoming coarser, stable operation can be achieved without any particular problems even if the heating temperature is slightly high. Therefore, if this is tempered at a temperature below Ac ₁ point, an extremely fine tempered martensite structure or an extremely fine mixed structure of tempered martensite and tempered low-temperature bainite can be obtained; (c) m. After the (m-1)th quenching process as an integer of 2 or more, and before the heating for the mth quenching process, a tempering process (hereinafter referred to as a rough ten bar) is performed to prevent cracks due to placement, etc.
(d) When one or more of Cu, Cr, V, Mo, and W are further added to the steel, the strength of the steel is further improved; When one or more of these elements is added, inclusions in the steel become spheroidized and the steel is cleaned, resulting in an improvement in toughness, and when a small amount of B is added, the strength and toughness of the steel are improved. to be further improved. This invention was made based on the above findings, and includes C: 0.15-0.45% (hereinafter, % is a weight percentage), Si: 0.05-0.10%, Mn: 0.3-2.0%, Al: 0.01-0. Contains 0.10%, one or both of Nb and Ti: 0.005 to 0.150%, and if necessary, the first category...Cu: 0.05 to 0.50%, Cr: 0.05 to 2.00%, V: 0.01 to 0.15% , one or two types of Mo and W: Mo+1/2W 0.05-1.20%, 2nd category... Ca: 0.001-0.050%, rare earth elements: 0.001-0.050%, 3rd category... B: 0.0005-0.0050%, A steel that also contains one or more of the following, Fe and unavoidable impurities: the remainder, and the content of P and S in the impurities is P: 0.025% or less, S: 0.015% or less, respectively. After hot rolling, Ac ₃ points ~ [Ac ₃ points +
200℃] to quench it from the austenitic state, followed by Ac ₃ points ~ [Ac ₃ points + 200
The process of heating to a temperature range of [℃] and then quenching is repeated one or more times, or tempering (rough tempering) at a temperature below Ac ₁ point and temperature range of Ac ₃ points to [Ac ₃ points + 200℃] The process of heating and quenching is repeated one or more times in this order, and then again.
By performing tempering treatment at a temperature of Ac ₁ point or less, it is characterized by obtaining a strong steel that exhibits high strength and excellent toughness even in low-temperature environments such as extremely cold regions. In addition, in the method of the present invention, the austenitizing heating temperature during the second and subsequent n-th quenching is preferably lower than the austenitizing heating temperature during the (n-1)th quenching. By doing so, the structure of the steel becomes finer and more uniform, and the toughness is further improved. In addition, when performing rough tempering to prevent cracks due to aging, etc., the rough tempering conditions are A ₁ = T (A ₂ + logt) [where, T: rough tempering temperature (〓) t: holding time at rough tempering temperature ( hr), A ₂ = 22-4 x C (%) - 10 x Nb (%) - 10 x Ti
(%)] It is preferable to set the setting so that A ₁ calculated using the formula satisfies A ₁ 19.0×10 ^3. By doing so, the collapse of martensite laths and low-temperature bainitic laths due to rough tempering is minimized. This makes it possible to obtain a finer structure in the mth quenching than in the (m-1)th quenching. Furthermore, if the heating in the first quenching is performed at a slow heating rate like normal electric furnace heating, then quenching is performed with rapid heat treatment, the grain structure becomes even finer. It goes without saying that this can be achieved and the effect of improving toughness is significant. Next, in the method for manufacturing strong steel of the present invention,
The reason why the chemical composition of the steel and the heat treatment conditions are limited as described above will be explained. A. Chemical composition of steel C. C component is indispensable for increasing the hardenability and strength of steel, as well as refining the grain, but if its content is less than 0.15%, it will not work properly. At a heating rate of The C content was set at 0.15 to 0.45% since it would lead to deterioration. Si The Si component is effective as a deoxidizing agent for steel, and also has the effect of increasing strength and hardenability, but if its content is less than 0.05%, the desired effect cannot be obtained. On the other hand, if the Si content exceeds 1.00%, the toughness will deteriorate, so the Si content should be reduced to 0.05%.
~1.00%. Mn The Mn component has the effect of improving hardenability, improving strength and toughness, and deoxidizing the steel, but if its content is less than 0.3%, the desired effects cannot be obtained; If the Mn content exceeds the Mn content, the toughness will deteriorate, so the Mn content is set at 0.3 to 2.0%. Al The Al component is added to stabilize the deoxidation of steel, homogenize it, and refine the grains, but if its content is less than 0.01%, the desired effect cannot be obtained; If the Al content exceeds 0.10%, the deoxidizing effect will be saturated, and the increase in inclusions will also lead to the occurrence of flaws and deterioration of toughness, so the Al content was set at 0.01 to 0.10%. Nb and Ti Nb and Ti components have the effect of increasing the strength of steel,
In addition to the effect of increasing temper softening resistance, there is an equal effect of refining the structure, but if the total content of one or two of these elements is
If it is less than 0.005%, the desired grain refinement cannot be achieved even if the quenching treatment is repeated two or more times, especially if the heating rate is slow, and on the other hand, the total content of one or two of these elements cannot be achieved. The amount
If it exceeds 0.150%, not only will it not be possible to obtain any further improvement in the above-mentioned action, but it will also cause deterioration of toughness.
The content of the species or two species was determined to be 0.005 to 0.150%. P and S It is preferable that the P and S content be as small as possible in order to improve the toughness of the steel, but considering the manufacturing cost of the steel, the upper limit of P is set to 0.025%,
The upper limit of S was set at 0.015%. Cu, Cr, V, Mo, and W These ingredients have the effect of improving the strength of steel, so one or more of them may be added as necessary.The details of each element are described below. The characteristics and reasons for limiting the content will be explained. (i) Cu Cu component is a preferable element that increases the strength of steel without significantly impeding its toughness, but if its content is less than 0.05%, the desired effect cannot be obtained;
Cu content exceeding 0.05 will cause deterioration of hot workability.
It was set at ~0.50%. (ii) Cr The Cr component has the effect of increasing the hardenability, strength, and temper softening resistance of steel, but if its content is less than 0.05%, the desired effect cannot be obtained;
Since Cr content exceeding 2.00% causes deterioration of toughness, the Cr content was set at 0.05 to 2.00%. (iii) Mo and W Both Mo and W components have the same effect of increasing hardenability and strength, and increasing temper softening resistance, but W
The atomic weight is approximately twice that of Mo, and in terms of effectiveness, the Mo content is half that of W, making them exactly equal. And Mo+1/2W
If the value of is less than 0.05%, the desired effect cannot be obtained, and even if Mo + 1/2W exceeds 1.20% and one or more of Mo and W is contained, the strength increasing effect is saturated, and This may lead to deterioration of toughness.
The content of one or more types of Mo and W
Mo + 1/2W was set at 0.05 to 1.20%. (iv) V The V component has the effect of increasing the strength of steel and the resistance to temper softening, but if its content is less than 0.01%, the desired effect cannot be obtained. On the other hand, if the V content exceeds 0.15%, it will lead to deterioration of toughness.
The content was set at 0.01-0.15%. Ca and rare earth elements These components both spheroidize inclusions in the steel and clean the steel, improving toughness in the direction perpendicular to the rolling direction and reducing the anisotropy of the steel. If necessary, one or more types of additives can be added as needed, but if the content is less than 0.001%, the desired effect cannot be obtained; on the other hand, if the content is less than 0.001%, If the content exceeds 0.001 to 0.050, the toughness improvement effect will not only be saturated, but also nonmetallic inclusions such as oxides will increase and the cleanliness of the steel will decrease. %. Note that it is a practically preferable means to add the rare earth element in the form of metal. B Component B has the effect of improving the hardenability of steel and improving its strength and toughness, so it is an element that can be added as necessary, but if its content is less than 0.0005%, it will not have the desired effect. on the other hand.
The B content was set at 0.0005 to 0.0050% since no further improvement effect would be brought about if the B content exceeded 0.0050%. In addition, in the case of B-treated steel, the N content (%) in the steel is [2.5 x B (%) - 1.5 x
10 ^-3 ] or less, coarse borocarbide may precipitate during tempering at Ac ₁ point or less, making it impossible to obtain steel with the desired high toughness. 3xB
(%) - 1.2 x 10 ^-2 ] or more, the hardenability improving effect of B will not be fully exerted, and there is a risk of deterioration of strength and toughness. It is desirable to set the following limits: 10 ^-3 <N (%) <3 x B (%) - 1.2 x 10 ^-2 . In addition, in this case, the steel contains 0.005% or more Ti
If it does not contain
It is desirable to set the temperature to 1075℃ or higher. B Heat Treatment Conditions In the method of the present invention, the steel constructed as described above is melted, rolled into thick plates, shaped steel, steel pipes, etc. using a normal method, and then subjected to a specific heat treatment. , the heat treatment conditions are as follows. Quenching conditions First, hot-rolled material is heated to Ac ₃ points or more.
After heating to a temperature range of [Ac ₃ points + 200℃] to completely austenite the structure, quenching with an appropriate cooling medium to create a low-temperature transformed structure is repeated two or more times. Naturally, if the heating temperature is less than Ac ₃ points, austenitization cannot be achieved.On the other hand, if the heating temperature exceeds [Ac ₃ points + 200℃], the austenite crystal grains will become coarse, making it difficult to apply the treatment of the present invention. Even if it is twisted, it becomes impossible to obtain a desired fine structure. Furthermore, setting the heating temperature to below [Ac ₃ points + 200° C.] suppresses the coarsening of austenite grains, which has the secondary effect of reducing susceptibility to quench cracking during quenching. As described above, by limiting the heating temperature during quenching in this way, even at a slow heating rate such as electric furnace heating, ultrafine low-temperature transformation can be achieved by repeating heating quenching two or more times. structure, and the toughness can be greatly improved. As mentioned earlier, it is preferable that the heating during the second and subsequent quenching steps be lower than that of the previous quenching.This will result in an even finer and more regular grained structure, which will improve the performance of the steel material. can be improved. Tempering Conditions If the microstructure obtained by the above-mentioned quenching treatment is finally tempered at a temperature below Ac ₁ point, the desired strength and toughness can be imparted to the steel. In this case, if the tempering temperature exceeds the Ac ₁ point, the strength of the steel material changes significantly and the toughness deteriorates, so the temperature was set to be below the Ac ₁ point. As mentioned earlier, the temperature of rough tempering is A ₁ = T (A ₂ + logt) [where T: rough tempering temperature (〓) t: holding time at rough tempering temperature (hr), A ₂ = 22 −4×C(%)−10×Nb(%)−10×Ti
(%)] It is preferable to set it so that A ₁ calculated by the formula satisfies A ₁ 19.0×10 ³ . Next, examples of the present invention will be specifically explained in comparison with comparative examples. Example 1 First, steels 1 to 62 having the compositions shown in Table 1 were produced by a conventional method. Next, after hot rolling these, quenching and tempering treatments were performed under the conditions shown in Table 2. The yield point, tensile strength, and Charpy fracture surface transition temperature of the obtained steel plates were measured, and the results are also shown in Table 2. From the results shown in Table 2, it is clear that steel whose chemical composition and heat treatment conditions are within the range of the present invention have excellent strength and toughness, and have a good balance. , or comparative steels whose heat treatment conditions are outside the scope of the present invention are clearly inferior in the above properties. Example 2 Steels 45 and 46, which are the steels subject to the present invention in Table 1 above
After being quenched and tempered under the conditions shown in Table 3, the strength and toughness were measured. The results thus obtained are also shown in Table 3. In Table 3, "comparative method" refers to methods marked with * that do not satisfy the heat treatment conditions of the present invention.

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[Table] (Note) * indicates that the conditions of the present invention are not met.

[Table] It is a thing. From the results shown in Table 3, it is clear that according to the method of the present invention, steel materials with an extremely excellent balance of strength and toughness can be obtained. On the other hand, it is also clear that if the heat treatment conditions are outside the scope of the present invention, only steel materials with inferior toughness will be obtained. Example 3 Steel 45, which is the steel subject to the present invention in Table 1 above, was quenched under the conditions shown in Table 4, and
The austenite grain size number (ATSM No.) was measured. The obtained results are also shown in Table 4. The results shown in Table 4 also show that only by repeating quenching two or more times as in the method of the present invention, an extremely fine steel structure can be achieved and a steel material with excellent toughness can be obtained. Example 4 Steel 23, which is the steel subject to the present invention in Table 1 above, was
Under the conditions shown in the table, quenching and tempering were performed with rough tempering in between, and the strength and toughness were measured.

[Table] Established. The obtained results are also shown in Table 5. From the results shown in Table 5, it is clear that steel materials with excellent strength and toughness can be obtained even if rough tempering is performed between each quenching treatment to prevent cracking, etc. It can also be seen that if the rough tempering conditions are set to A ₁ 19.0×10 ³ , a steel material with an even better balance of strength and toughness can be obtained. As described above, the present invention does not require the addition of expensive elements such as Ni, nor does it require special equipment or troublesome work, and is suitable for large structures used in extremely cold regions. Industrially useful effects are brought about, such as the ability to easily produce strong steel at low cost.

Claims (1)

[Claims] 1. In weight percentage: C: 0.15-0.45%, Si: 0.05-1.00%, Mn: 0.3-2.0%, Al: 0.01-0.10%, one or both of Nb and Ti: 0.005 ~0.150%, with the remainder consisting of Fe and unavoidable impurities, and the contents of P and S in the unavoidable impurities are P: 0.025% or less, S: 0.015% or less, respectively. After hot rolling, it is heated to a temperature range of ₃ points Ac to [ ₃ points Ac + 200℃] to quench it from an austenitic state, and then heated to a temperature range of ₃ points Ac to [ ₃ points Ac + 200℃]. A method for producing strong steel, comprising: repeating heating and then quenching one or more times, and then performing tempering at a temperature below Ac ₁ point. 2 Contains, in weight percentage, C: 0.15-0.45%, Si: 0.05-1.00%, Mn: 0.3-2.0%, Al: 0.01-0.10%, one or both of Nb and Ti: 0.005-0.150%. In addition, Cu: 0.05 to 0.50%, Cr: 0.05 to 2.00%, V: 0.01 to 0.15%, one or two of Mo and W: 0.05 at Mo+1/2W
~1.20%, Ca: 0.001~0.050%, rare earth elements: 0.001~0.050%, B: 0.0005~0.0050%, and the rest is
After hot rolling steel, which has a composition consisting of Fe and unavoidable impurities, and the contents of P and S in the unavoidable impurities are P: 0.025% and S: 0.015%, Ac ₃ points ~ The process of heating to a temperature range of [Ac ₃ points + 200℃] and quenching from the austenite state, then heating and quenching to a temperature range of Ac ₃ points to [Ac ₃ points + 200℃] is repeated one or more times. A method for producing strong steel _, characterized by: 3 Contains, in weight percentage, C: 0.15-0.45%, Si: 0.05-1.00%, Mn: 0.3-2.0%, Al: 0.01-0.10%, one or both of Nb and Ti: 0.005-0.150%. A steel having a composition in which the remainder is Fe and unavoidable impurities, and the contents of P and S in the unavoidable impurities are P: 0.025% or less and S: 0.015% or less, respectively, was hot rolled. After that, it is heated to a temperature range of Ac ₃ points to [Ac ₃ points + 200℃] to harden from the austenitic state, followed by tempering at a temperature of Ac ₁ point or less, and Ac ₃
Toughness characterized by: heating to a temperature range of Ac ₃ points + 200℃, followed by quenching, repeated one or more times in this order, and then tempering again at a temperature below Ac ₁ point. Method of manufacturing steel. 4 Contains, in weight percentage, C: 0.15-0.45%, Si: 0.05-1.00%, Mn: 0.3-2.0%, Al: 0.01-0.10%, one or both of Nb and Ti: 0.005-0.150%. In addition, Cu: 0.05 to 0.50%, Cr: 0.05 to 2.00%, V: 0.01 to 0.15%, one or two of Mo and W: 0.05 at Mo+1/2W
~1.20%, Ca: 0.001~0.050%, rare earth elements: 0.001~0.050%, B: 0.0005~0.0050%, and the rest is
Ac ₃ It is heated to a temperature range from point to [Ac ₃ point + 200℃] to harden from the austenitic state, followed by tempering at a temperature below Ac ₁ point, and then to Ac _3.
A strong steel characterized by: heating and quenching to a temperature range from point to [Ac ₃ point + 200°C], repeated one or more times in this order, and then tempering again at a temperature below Ac ₁ point. manufacturing method.